The present invention relates generally to methods and apparatus for treating eating disorders by application of modulating electrical signals to a selected cranial nerve, nerve branch or nerve bundle, and more particularly to techniques for treating patients with overeating disorders, especially obese patients, by application of such signals unilaterally or bilaterally to the patient's vagus nerve with one or more neurostimulating devices.
Increasing prevalence of obesity is one of the most serious and widespread health problems facing the world community. It is estimated that, currently, about 6% of the total population of the United States is morbidly obese and a much larger percentage is either obese or significantly overweight. Morbid obesity is defined as having a body mass index of more than forty, or, as is more commonly understood, being more than one hundred pounds overweight for a person of average height. Aside from what may be an epidemic of obesity, it is believed by many health experts that obesity is one of the first two leading causes of preventable deaths in the United States, either ahead of or just behind cigarette smoking. or not that is an accurate assessment, studies have indicated that morbid obesity dramatically increases health care costs. It is a major cause of adult onset diabetes in the United States, up to approximately eighty percent of the cases. It may be a leading factor in as many as ninety percent of sleep apnea cases. Obesity is also a substantial risk factor for coronary artery disease, stroke, chronic venous abnormalities, numerous orthopedic problems and esophageal reflux disease. Researchers have documented a link between obesity, infertility and miscarriages, as well as post menopausal breast cancer.
The classical treatment option for obese people combines nutritional counseling with exercise and education, but has demonstrated relatively little long term success. Liquid diets and pharmaceutical agents can bring about acute, but not lasting weight loss. Surgical procedures for either gastric restriction or malabsorption in cases of severe obesity have shown the greatest success long-term, but are major surgery that can lead to emotional problems, and have their share of failures (e.g., Kriwanek, “Therapeutic failures after gastric bypass operations for morbid obesity,” Langenbecks Archiv. Fur Chirurgie, 38(2): 70-74, 1995).
U.S. Pat. No. 5,263,480 to J. Wemicke et al., assigned to the same assignee as the present application, discloses treatment for eating disorders including obesity and compulsive overeating disorder by selectively applying modulating electrical signals to the patient's vagus nerve, preferably using an implanted neurostimulator. Modulating signals may be used to stimulate vagal activity to increase the flow of neural impulses up the nerve, or to inhibit vagal activity to block neural impulses from moving up the nerve, toward the brain, for producing excitatory or inhibitory neurotransmitter release.
Both of these cases of modulating the electrical activity of the vagus nerve have been termed vagus nerve stimulation, or VNS. The '480 patent theorized that VNS could be used for appetite suppression by causing the patient to experience satiety, a sensation of “fullness” of the stomach which would result in decreased food consumption and consequent weight reduction. For example, the stimulus generator of the neurostimulator is implanted in a convenient location in the patient's body, attached to an electrical lead having a nerve electrode implanted on the vagus nerve or branch thereof in the esophageal region slightly above the stomach. If the patient's food consumption over a given period exceeded a predetermined threshold level, detected and measured for example by sensing electrodes implanted at or near the esophagus, the stimulus generator is triggered to apply VNS and thereby induce satiety. Alternatively, VNS is applied periodically during the patient's normal waking hours except in periods of prescribed mealtimes, or is applied as a result of patient intervention by manual activation of the stimulus generator using external magnet control. Patient intervention assumes a patient with an earnest desire to control his or her eating behavior, but normally lacking will power to control the compulsive behavior without the support of VNS.
Like most of the pairs of cranial nerves, the tenth cranial nerve, the vagus, originates from the brain stem. It passes through foramina of the skull to parts of the head, neck and trunk. The vagus is a mixed nerve, with both sensory and motor fibers, the sensory fibers being primary and attached to neuron cell bodies located outside the brain in ganglia groups, and the motor fibers attached to neuron cell bodies located within the gray matter of the brain. The vagus, as a cranial nerve, is part of the peripheral nervous system or PNS whose nerves branch out from the central nervous system (CNS) to connect the CNS to other body parts. Somatic fibers of the cranial nerves are involved in conscious activities and connect the CNS to the skin and skeletal muscles, while autonomic fibers of these nerves are involved in unconscious activities and connect the CNS to the visceral organs such as the heart, lungs, stomach, liver, pancreas, spleen, and intestines.
The motor fibers of the vagus nerve transmit impulses to the muscles associated with speech and swallowing, the heart, and smooth muscles of the visceral organs of the thorax and abdomen. In contrast, its sensory fibers transmit impulses from the pharynx, larynx, esophagus and visceral organs of the thorax and abdomen. The vagus is split into left and right branches, or left and right vagi, which run respectively through the left and right sides of the neck and trunk. It is the axial portion of the body, which includes the head, neck and trunk with which we are primarily concerned in respect of the present invention. The ventral cavity of the axial portion contains visceral organs and includes the thoracic cavity and the abdominopelvic cavity, which are separated by the diaphragm, a broad thin muscle. Visceral organs in the thoracic cavity include the right and left lungs, the heart, the esophagus, the trachea and the thymus gland. Below the diaphragm, in the abdominopelvic cavity and specifically the upper abdominal portion or abdominal portion, the visceral organs therein include the stomach, liver, spleen, gall bladder, and majority of the small and large intestines.
The vagus nerve is the dominant nerve of the gastrointestinal (GI) tract, the right and left branches or nerve afferents of the vagus connecting the GI tract to the brain. After leaving the spinal cord, the vagal afferents transport information regarding that tract to the brain. In the lower part of the chest, the left vagus rotates, becomes the anterior vagus, and innervates the stomach. The right vagus rotates to become the posterior vagus, which branches into the celiac division and innervates the duodenum and proximal intestinal tract.
The exact mechanisms leading an individual to satiety are not fully known, but a substantial amount of information has been accumulated. Satiety signals include the stretch of mechanoreceptors, and the stimulation of certain chemosensors (e.g., “A Protective Role for Vagal Afferents: An Hypothesis,” Neuroanatomy and Physiology of Abdominal Vagal Afferents, Chapter 12, CRC Press, 1992). These signals are transported to the brain by the nervous system or endocrine factors such as gut peptides (e.g., “External Sensory Events and the Control of the Gastrointestinal Tract: An Introduction” id. at Chapter 5). It has been demonstrated that direct infusion of maltose and oleic acid into the duodenum of rats leads to a reduction in food intake, and that the response is ablated by vagotomy or injection of capsaicin, which destroys vagal afferents. Introduction of systemic cholecystokinin also reduces intake in rats, and is ablated by destruction of vagal afferents.
While the vagus is often considered to be a motor nerve which also carries secretory signals, 80% of the nerve is sensory consisting of afferent fibers (e.g., Grundy et al., “Sensory afferents from the gastrointestinal tract,” Handbook of Physiology, Sec. 6, S. G., Ed., American Physiology Society, Bethesda, Md., 1989, Chapter 10).
The aforementioned '396 application discloses a method of treating patients for obesity by bilateral stimulation of the patient's vagus nerve (i.e., bilateral VNS) in which a stimulating electrical signal is applied to one or both branches of the vagus. The parameters of the signal are predetermined to induce weight loss of the patient. The signal is preferably a pulse signal applied at a set duty cycle (i.e., its on and off times) intermittently to both vagi. In any event, VNS is applied at a supra-diaphragmatic position (i.e., above the diaphragm) in the ventral cavity. The electrical pulse stimuli are set at a current magnitude below the retching level of the patient (e.g., not exceeding about 6 milliamperes (mA), to avoid patient nausea) in alternating periods of continuous application and no application. Pulse width is set at or below 500 microseconds (μs), and pulse repetition frequency at about 20-30 Hz. The on/off duty cycle (i.e., first period/second period of the alternating periods) is programmed to a ratio of about 1:1.8. The neurostimulator, which may be a single device or a pair of devices, is implanted and electrically coupled to lead(s) having nerve electrodes implanted on the right and left branches of the vagus.